1. Field of the Invention
The present invention relates to magnetic recording medium direction sensing.
2. Description of the Prior Art
In order to edit video signals rapidly and precisely it has been proposed to record an address signal on a magnetic tape in addition to the video and audio signal. Various coded address signals have been proposed, but the Socity of Motion Picture and Television Engineers (SMPTE) time code signal is recommended as an American Nation Standard and the European Broadcasting Union (EBU) time code signal is recommended as a standard code for 625 line/50 field television tape recordings. These two time code signals are recorded on a magnetic tape along a longitudinal track, and readout of the signals can be achieved at tape speeds from slow to high speed. However, in cases where the magnetic tape is stopped or is transported at very low speed, the reproduction of the time code signals becomes impossible. In fact, on editing a magnetic tape using a video tape recorder (VTR), it is very advantageous to be able to choose individual frames presented visually at very low tape speeds, but with the above-mentioned time code signals the use of very low tape speeds means that the address of a chosen frame cannot readily be known.
In the above-mentioned application there is described an address signal which is inserted into the video signal itself. This address signal is called a vertical interval time code (VITC) signal. The VITC signal is recorded on the magnetic tape as part of the video track, so that the VITC signal can always be reproduced by a rotary magnetic head regardless of the tape transport speed and direction.
Time code signals, such as the SMPTE signal, recorded longitudinally on a magnetic tape have a synchronising word to indicate the tape transport direction, so that the address signal can be correctly decoded. Therefore, after reproducing one word of an SMPTE signal it is immediately possible to produce the next address to be reproduced merely by adding or subtracting one to or from the present address.
The VITC signal on the other hand is scanned by a rotary magnetic head which is always moving in the same direction relative to the video tracks in which the video signal is recorded on the magnetic tape, so the VITC signal has no synchronising word therein. It is not therefore possible to determine the tape transport direction from the VITC signal corresponding to one video track. This is a disadvantage, particularly where it is required to generate an address signal by calculation. This requirement may arise as follows:
Firstly, to stop the magnetic tape at a predetermined address exactly, a calculated address signal is used to determine the address of the video track next before the video track having the predetermined address.
Secondly, if drop-out results in loss of the VITC signal in the reproduced signal, a VITC signal calculated from a preceeding VITC signal not lost due to drop-out can be used in place of the lost VITC signal.
Thirdly, to avoid problems caused by drop-out, it is usual, when transcribing from a playback VTR to a recording VTR, to insert a new, calculated, VITC signal in the recorded video signal.